Variantly: "and to serve Him": This is prayer. — But perhaps it is service, literally. It is, therefore, written "with all your heart." Now is there "service" in the heart? What, then, is the intent of "and to serve Him with all your heart?
Prayer. Similarly, David said (Psalms 141:2) "Let my prayer stand as incense before You; the lifting of my hands, as an evening offering." And it is written (Daniel 6:11) "And when Daniel learned that the writing had been inscribed, he went home. He had windows open in his upper storey, facing Jerusalem, and three times a day he fell to his knees and gave thanks before his G-d, exactly as he used to do before this."
And (Ibid. 21) "And when he (the king) drew near to the pit, to Daniel, he cried out in a sad voice. The king answered and said to Daniel: 'Daniel, servant of the living G-d, was your G-d, whom you serve continually, able to save you from the lions?'" Now was there (Temple) service in Bavel? What, then, is the intent of "and to serve Him"? Prayer. Just as the sacrificial service is called "service," so, prayer.